This invention relates to nondestructive examination of boiler tubes and, in particular, to the ultrasonic detection and measurement of the thickness of oxide scale on the inner cylindrical surface of boiler tubes.
Scale deposits on the waterside of boiler tubes undesirably increase heat transfer resistance and have long been recognized as a cause of boiler tube failure. In the electric utility industry, a metallurgical technique is typically employed to measure scale thickness to predict the remaining life of high pressure boiler tubes.
With the metallurgical measuring technique, a sample of the boiler tube is removed from the boiler and a new section of tubing is installed in its place. The sample is sent to a metallurgical laboratory. A nickel coating is applied to the sample to prevent the scale from breaking loose during sample preparation. Next, small sections are cut from the tube sample, mounted, polished, etched and examined under a metallurgical microscope. Once measured, the thickness of the scale is used in an algorithm to predict the remaining life of the boiler tube.
Metallurgical examination provides a very accurate measure of scale thickness. However, such examinations inconveniently require physical removal and replacement of boiler tube sections and analysis of the removed tube sections at laboratory facilities each time a scale thickness measurement is desired. The overall procedure is very time consuming and expensive as it directly relates to downtime, i.e. time loss in ability to generate power.
At the present time, there are no known commercialized nondestructive techniques for measuring or detecting scale on the inner cylindrical surface of boiler tubes. The recovery and utility boiler industries have expressed a need for such techniques.
Ultrasonics are presently utilized, in the inspection of boiler tubes, for example, for detecting surface and subsurface flaws and for the measurement of the thickness of a material or the distance to a flaw. Hence, the development of an in situ, nondestructive boiler tube scale measurement technique, particularly one which employs ultrasonics, would be highly desirable.